Modern mobile communications systems use multiple channels, closely spaced over an assigned frequency band. In order to avoid intermodulation products and spectral regrowth, both in and out of band, it is essential that RF power amplifier circuits used in these systems be highly linear. A high level of linearity is also required in single-channel transmitters which transmit a wideband, variable-envelope signal, such as a CDMA signal.
A major source of nonlinearity is distortion, which occurs due to nonlinear amplitude and phase response of the amplifier, particularly as power nears the saturation level. Third-order distortion nonlinearities typically give the strongest intermodulation products, but fifth- and even seventh-order products can be significant. Since a typical cellular communications band has a spectral width of around 25 MHz, high-order intermodulation products in a wideband base station amplifier with large channel spacing can create distortion over a band that is more than 150 MHz wide.
One method of correcting for amplifier distortion, and thus improving linearity, is predistortion, in which a controlled, nonlinear distortion is applied to the amplifier input signals. Predistortion circuitry is designed to give nonlinear amplitude and phase characteristics complementary to the distortion generated by the amplifier itself, so that ideally, the distortion is canceled in the amplifier output over the entire signal bandwidth. A feedback connection is generally provided from the amplifier output to the predistortion circuitry, for use in adjusting predistortion coefficients for optimal linearization. Predistortion is often applied to baseband signals, as described, for example, in U.S. Pat. No. 4,291,277, which is incorporated herein by reference. The predistorted signals are then upconverted and fed to the power amplifier. Predistortion may also be combined with other methods of linearization, such as feedforward error correction, as described in U.S. Pat. No. 5,760,646, which is likewise incorporated herein by reference.
Various schemes have been proposed for digital-domain predistortion of the baseband signals. Because of the very high bandwidth of the intermodulation products, as mentioned above, extremely fast, wideband processing circuitry has been required in order to compensate effectively for distortion without causing new problems such as aliasing. The required sampling rate is particularly high when the power amplifier has a significant level of high order (fifth or seventh order) intermodulation response. For example, U.S. Pat. No. 4,700,151, which is incorporated herein by reference, describes a predistortion system that operates on baseband signals. The signals are sampled and then interpolated to generate samples having a higher sample rate, thus providing an extended bandwidth as required for effective predistortion. U.S. Pat. No. 5,650,758, also incorporated herein by reference, describes a pipeline architecture for a wideband digital predistortion circuit. Other predistortion schemes are described in an article by Cavers, entitled "Amplifier Linearization Using a Digital Predistorted with Fast Adaptation and Low Memory Requirements," published in IEEE Transactions on Vehicular Technology, vol. 39, no. 4 (November 1990), pages 374-382, which is incorporated herein by reference.